1. Field of the Invention
The present invention relates to an imaging apparatus in which a gradation (gamma: γ) property (gradation property curve) can be changed, and particularly to an imaging apparatus in which the gradation property can be changed and exposure control is improved.
Moreover, the present invention relates to an imaging apparatus in which an image pickup device such as CCD is incorporated, and particularly to an imaging apparatus in which sensitivity is enhanced by addition of pixels.
2. Description of the Related Art
In recent years, an imaging apparatus has intensively been developed in which a still picture or an image of a photographic subject is taken by an image pickup device such as CCD and converted to an image signal. In such known imaging apparatus, a gradation property (output signal level property corresponding to photographic subject luminance) can be changed. Concretely, for example, in a broadcasting TV camera or a so-called industrial camera (ITV), the gradation property is set to one of a standard set mode (γ=0.45) and a linear property set mode (γ=1) by so-called “gamma change”.
Here, a γ (gamma) value is an index. Dissociation from a linear input/output property is noted as well known, and a relation between input x and output y is approximately represented by an equation y=a×xγ+b (a, b are constants). Therefore, when γ=1, the input and output are in a proportional relation. Additionally, since b corresponds to a pedestal or an offset, it is often taken into consideration independently of the γ value.
A value γ=0.45 is a standard system value in broadcasting, and selected as a value for obtaining a general high quality (visually satisfactory image quality for a purpose of appreciation) in consideration of the gradation property of a reproduction system. On the other hand, γ=1 is selected as a value for removing an error factor attributed to a gradation conversion circuit for a purpose of measurement or in camera adjustment.
An automatic exposure control is often used for the camera described above. Generally, a control object of the automatic control is an output from the image pickup device, that is, a linear signal before inputted to the gradation conversion circuit usually called “γ conversion circuit”. According to the control method, optimum exposure control directly corresponding to a dynamic range of the image pickup device can be realized.
In recent years, an electronic still camera which is one type of the electronic imaging apparatus described above and which is capable of recording a still picture has come into wide use in the name of a “digital camera”, and there has been a need for a change of the gradation property in a meaning different from that of a broadcasting TV camera or an industrial camera. That is, there has been a demand for a digital camera in which an optimum gradation can selectively be set in accordance with user's taste or scene situation in order to obtain a high-quality picture comparable to a picture on a silver-halide or silver-salt film.
On the other hand, for the digital camera intended to take the picture corresponding to the silver-halide picture, a natural requirement is that a person skillful in taking the silver-halide picture can satisfactorily use the digital camera. Therefore, specifications and operability (so-called handiness) equivalent to those of the silver-halide picture are required in background. To this end, for example, an attempt to indicate “sensitivity” of the digital camera in so-called “ISO indication” similarly as a conventional silver-halide film has been carried out.
One attempt is described in “Television Society Technical Report/Yoshida: Study on Digital Camera Sensitivity (Speed) Indicating Method: ITE Technical Report Vol. 20, No. 58, PP. 85 to 90, CE'96-25 (November, 1996)”. In a measuring method used in the sensitivity indicating method, the sensitivity is defined with “an exposure amount which gives a predetermined value (proposed value 106.5/255) defined in an intermediate range (satisfactory gradation reproduction range) of a digital value recorded by the digital camera”. With the camera having the same “sensitivity” obtained by the measuring method, the same digital signal output is obtained during photography with the predetermined exposure amount for a measurement reference.
Additionally, another discussion is necessary for judging whether this proposal can be said to correspond to the so-called “ISO indication”. However, the “sensitivity” itself proposed in the document is “an index indicating the exposure amount necessary for obtaining an image with the same brightness (output level)”, and is therefore significant. Moreover, in the following description, the “sensitivity” indicates such sensitivity as described in the document (the predetermined value is indefinite) unless otherwise specified.
When the gradation property is changed in accordance with the user's taste and scene situation and the gradation is selected as described above, even with a constant gain of an imaging system, the sensitivity changes with the gradation property change. Therefore, a problem is that the obtained output level changes.
This respect will be described in more detail. Since the gradation property is changed, it is originally impossible to obtain an equal output level over a whole luminance distribution of the photographic subject. However, considering from an average or typical portion of the luminance distribution of the photographic subject, in other words, considering only from a main photographic subject, it is preferable that the output level for the subject should not change even with the changed gradation property. Additionally, in the conventional digital camera, since these are not considered, the output level disadvantageously changes largely as described above.
Moreover, concerning sensitivity enhancement, even when the imaging apparatus is a dynamic imaging apparatus or a still imaging apparatus, adjacent pixel information of the image pickup device is added, for example, signals of four pixels in total of two pixels in a vertical direction and two pixels in a horizontal direction are added, then resolution is deteriorated, but the sensitivity can be enhanced.
Known examples of an adding method include an outer digital adding system, device inside analog adding system, and the like. The outer digital adding system comprises: reading a pixel signal from the image pickup device for each pixel in a usual method, subsequently subjecting the signal to A/D conversion, and adding the pixel signals in a digital system. On the other hand, in the device inside analog adding system, transfer drive of a CCD image pickup device is devised in the image pickup device, and charges are added in a transfer path. As a result of comparison of the systems, the analog adding system is superior in that a frame rate can also be enhanced.
Additionally, when this technique is actually applied to the electronic camera, the sensitivity can be enhanced, but a noise increases with the sensitivity enhancement. Beyond simple resolution deterioration because of a decrease of the number of pixels, image quality deterioration further occurs.
In this respect, for example, it is also described in the conventional publication that fourfold-sensitivity can be obtained in addition of four pixels. However, this cannot be said to be correct in consideration about the noise. Because it is known that noise improvement effect in the pixel addition, that is, SN enhancement is proportional to a square root of an added number of the pixels in respect to a statistic property of random noise. In the addition of four pixels, only double SN (i.e., 6 dB) can be expected. Therefore, during imaging while the image quality, that is, the noise level is kept, the imaging with double sensitivity, that is, ½ of the standard exposure amount is a limitation. Supposing that the image is taken with a ¼ exposure amount and four pixels are added in order to obtain the fourfold-sensitivity, a signal component S is ¼×4=once, noise N is 1×√{square root over (4)}=twice, and SN is deteriorated by 6 dB.
Then, addition of four pixels and exposure with the ½ exposure amount are considered. In an analog adding system in the digital camera, there is a problem of saturation in an A/D converter. That is, if a maximum output level of a horizontal transfer path or an image pickup device output amplifier as a position for executing the addition is not limited, it is necessary to handle a charge amount for ½×4=2 pixels, that is, double output voltage in the addition of four pixels with the ½ exposure amount. Therefore, when the conventional usual A/D converter corresponding to the voltage for one pixel device is used as it is, an A/D input voltage exceeds A/D quantized maximum voltage, and the signal is clipped.
In this case, in order to prevent the clip during the quantization in the A/D converter, it is necessary to set a maximum quantization level (Dmax) of A/D to be twice the conventional level. With this setting, the problem of saturation can securely be avoided. However, in a usual non-addition time, the maximum value of CCD output signal is Dmax/2, and a larger digital bit is wasted. In other words, a relative quantized error disadvantageously becomes double from an ideal state.
Additionally, to avoid the problem, the A/D converter quantized error itself may be reduced. In other words, the A/D converter with a larger number of bits than an originally necessary number of bits is necessary, and cost increase is unfavorably caused.
In the conventional art, the addition of pixels is carried out in this manner in order to enhance the sensitivity of the imaging apparatus. However, in the analog adding system, since the A/D input voltage exceeds the A/D quantized maximum voltage, the signal is clipped. There is a problem that the image quality is deteriorated.